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fs: fix write to unallocated block within file range

This case occurs with Twrite offset >= 8192 followed by Twrite
offset < 8192.

--- a/fs.c

+++ b/fs.c

@@ -284,7 +284,7 @@

 static int

 writeb(Fid *f, Msg *m, Bptr *ret, char *s, vlong o, vlong n, vlong sz)

 {

-	char buf[Kvmax];

+	char *e, buf[Kvmax];

 	vlong fb, fo;

 	Blk *b, *t;

 	Bptr bp;

@@ -303,14 +303,18 @@

 		return -1;

 	t = nil;

 	if(fb < sz && (fo != 0 || n != Blksz)){

-		if(lookup(f, m, &kv, buf, sizeof(buf), 0) != nil)

+		e = lookup(f, m, &kv, buf, sizeof(buf), 0);

+		if(e == nil){

+			bp = unpackbp(kv.v, kv.nv);

+			if((t = getblk(bp, GBraw)) == nil)

+				return -1;

+			memcpy(b->buf, t->buf, Blksz);

+			freeblk(f->mnt->root, t);

+			putblk(t);

+		}else if(e != Eexist){

+			werrstr("%s", e);

 			return -1;

-		bp = unpackbp(kv.v, kv.nv);

-		if((t = getblk(bp, GBraw)) == nil)

-			return -1;

-		memcpy(b->buf, t->buf, Blksz);

-		freeblk(f->mnt->root, t);

-		putblk(t);

+		}

 	}

 	if(fo+n > Blksz)

 		n = Blksz-fo;